With the pattern of “large-scale development and long-distance transportation” of clean energy power generation, a higher proportion of power energy will be sent from the external transmission channels, especially the direct current (DC) transmission channels to the load center. As a result, load-center power grids will surely develop into typical weak receiving-end power grids with limited local power supply. In order to ensure the power supply reliability and power quality of the weak receiving-end power grid, the weak receiving-end power grid must have the following three capabilities: 1) rapid dynamic reactive power support capability; 2) economic reactive power regulation capability; and 3) adequate inertia support capability.
The synchronous condenser based on synchronous machine can play a role in regulating the reactive power and supporting the local grid voltage through adjusting its excitation voltage. However, due to its large time constant of the exciting windings, the synchronous condenser cannot achieve rapid regulation of the dynamic reactive power. In addition, since the synchronous condenser is based on DC excitation and not driven by a prime mover, when the condenser is in operation, the rotate speed of the condenser must be maintained within a safe range near the synchronous speed. Thus, when the frequency disturbance occurs in the system, the inertia support that the synchronous condenser based on synchronous machine can release is very limited, namely, the demand of the weak receiving-end power grid for the inertia support cannot be met.
The Static Var Generator (SVG) based on fully-controlled power electronic device can achieve fast reactive power regulation. However, due to the converter's direct connection with the power grid, the actual capacity and regulation capability of the equipment are constrained by the device capacity, which results in high cost and poor economy. In addition, SVG cannot provide inertia support for the system due to its own operational characteristics.
If the traditional reactive power compensation equipment are used to enhance the three capacities required by the weak receiving-end power grid, we will inevitably face the problem of a large amount of repeated construction, and taking into account the very expensive cost of SVG and the weaker inertia support capacity of the synchronous condenser, such repeated construction must be unacceptable.
In general, the existing reactive power regulation devices cannot meet both the fast and economical requirements of the reactive power support as well as the considerable inertia support capability.